Field of the Invention
The invention relates to the field of implantable cardiac stimulation devices and, more especially, to implantable cardiac stimulation devices and methods of use of such devices to observe and evaluate timing of atrial events, for example for monitoring for onset of atrial fibrillation (AF) and/or the progression of heart failure (HF).
Description of the Related Art
Heart failure (HF) refers broadly to a variety of health ailments related generally to weakening and/or damage to the heart and characterized by a reduction in the mechanical ability of the heart to deliver an appropriate supply of blood. Heart failure can encompass an enlargement/dilation of the heart muscle, a degradation of the contractile properties of the heart, and/or a reduction in the synchrony among the heart chambers during cardiac contractions. Heart failure can also correspond to damage to or deterioration of heart valves and other structural conditions which reduce the cardiac output. Heart failure is also frequently found coincident with a variety of cardiac arrhythmias.
Heart failure can be of a varying degree of severity, ranging from the least severe where the HF condition may be detected upon clinical evaluation and wherein overt symptoms may only be noticed during strong physical exertion to the most severe conditions of HF, wherein the patient experiences severe symptoms even when fully resting. A variety of therapies are available to treat HF and the severity and progress of an HF condition is a valuable indicator for the patient's overall health status. Thus, it will be appreciated that being able to readily identify and characterize either the onset of an HF condition or the progression/status of a known HF condition can provide a valuable diagnostic tool to a clinician to provide more effective therapy to the patient.
A variety of examinations and observations can be utilized by a clinician to evaluate the existence or progression of an HF condition. A physical examination and interview of the patient can reveal, for example, edema and/or weight gain caused by fluid accumulation, which is a frequent symptom of HF. Shortness of breath is also a common symptom of HF and an interview of the patient and examination can reveal the severity of and conditions under which the shortness of breath occurs. An examination can also reveal a third heart sound (frequently referred to as S3) as well as a sound of fluid in the lungs during inspiration (rales), either of which are common symptoms of HF. A clinician may also observe enlargement of the jugular vein in the neck region (jugular venous distention) and/or enlargement of the liver (hepatomegaly). This may be coupled with a hepatojugular reflex wherein an enlarged liver which is subjected to manual pressure forces more blood into the jugular veins, causing them to become even more enlarged.
Several diagnostic tests are also useful in diagnosing HF, including chest x-rays which can reveal pulmonary edema, an enlarged heart, and pleural effusion. Electrocardiograms (EKGs) are also useful for their ability to detect the presence of a heart attack, cardiac ischemia, abnormal heart rhythms, and/or an enlarged heart. Echocardiograms are additional useful diagnostic tools which can determine the amount of blood ejected from the heart with each heartbeat, and more particularly, the proportion of blood ejected which is typically referred to as the ejection fraction. The ejection fraction is a useful way to quantitatively characterize the efficiency of the heart which is closely related to the presence or severity of a HF condition. For a normal healthy person, the ejection fraction typically is in the range of approximately 55 to 75%. A person suffering from HF would typically have a lower ejection fraction with a more depressed ejection fraction indicating a more severe HF condition. Echocardiograms can also diagnose particular causes of HF, including heart valve abnormalities, pericardial abnormalities, congenital heart disease, and/or an enlarged heart. Echocardiograms can also show if the contraction of the heart itself is abnormal, such as in wall motion abnormalities.
While these clinical observations and diagnostic tests offer valuable information for diagnosing the presence/progress of a heart failure condition, they suffer from the disadvantage of requiring the direct intervention of a highly trained clinician. The aforementioned patient observations require the training and judgment of a skilled clinician to accurately diagnose the patient observations. The aforementioned diagnostic tests, in addition to requiring the services of a skilled clinician to perform the tests, also typically require that the tests take place in a clinical setting. Diagnostic equipment such as chest x-ray and echocardiogram machines are large, complex, and relatively expensive pieces of equipment which are neither portable nor economical for the dedicated service of a single patient. Thus, the aforementioned observations and diagnostic tests are not suitable for frequent ongoing diagnosis of a patient's HF condition but rather are more suitable to serve patients at scheduled clinical appointments.
Another drawback to clinical evaluations is that emerging conditions can occur intermittently and may not manifest themselves during the limited clinical session, thus making proper diagnosis problematic. Early diagnosis and initiation of appropriate therapy can be very valuable in mitigating a new or changing condition. For example, monitoring intra-atrial conduction time (IACT) provides a surrogate for progression or remodeling such as LA dilation, LV functions and Mitral Valve Regurgitation (MR). Early identification of onset of atrial fibrillation (AF) as well as establishing the predominant cause of the AF can be very beneficial in mitigating the effects of the AF. AF refers to a rapid, generally chaotic, atrial arrhythmia with dramatically reduced pumping efficiency. Early detection of AF, for example at a paroxysmal stage, can enable early provision of therapy to prevent paroxysmal AF from possibly progressing to permanent AF. Establishing a predominant cause of a patient's AF is important as certain treatments for certain categories of AF are contraindicated for other categories of AF.
For example, the heart's natural rate control system includes inputs from two branches of the autonomic (involuntarily controlled) nervous system. The cardiac nerve provides a sympathetic branch of the autonomic nervous system; stimulation by the cardiac nerve will speed the heart rate up (adrenergically mediated). The vagus nerve provides a parasympathetic branch of the nervous system, wherein vagal activity slows the heart rate. These two physiologic controls respond to and balance the body's metabolic requirements, for example by speeding up blood flow during exercise or reducing it again afterwards. In a normal heart, the vagus nerve is active and provides a “vagal drive” to slow an otherwise faster intrinsic heart rate. Certain incidences of AF are considered to be largely vagally mediated as opposed to non-vagal or adrenergic AF. The importance of distinguishing AF which is vagally mediated from non-vagal AF is that, depending on the case, certain drug treatments that can be helpful in one case are in the other case unhelpful, and at worst harmful, to the patient's condition.